[FLINK-25222][tests] Remove NetworkFailureProxy used in Kafka tests

[fapaul]

What is the purpose of the change

We suspect that the NetworkFailureProxy is causing constant connectivity
problems to the brokers during testing resulting in either network
timeouts or corrupted results.
Since the NetworkFailureProxy is only used for testing the deprecated
FlinkKafkaProducer/Consumer we can safely remove it because we will not
add new features to the connectors.

Brief change log

• Remove NetworkFailureProxy and related tests
• Close leaking Admin Clients
• Await broker shutdown

Verifying this change

Only test code has changed and all existing tests still pass

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[flinkbot]

CI report:

• 999dcc9 Azure: SUCCESS

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Automated Checks

Last check on commit 999dcc9 (Tue Dec 14 09:07:36 UTC 2021)

Warnings:

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Review Progress

• ❓ 1. The [description] looks good.
• ❓ 2. There is [consensus] that the contribution should go into to Flink.
• ❓ 3. Needs [attention] from.
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• ❓ 5. Overall code [quality] is good.

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[alpreu]

@flinkbot run azure

[alpreu]

Looks good to me :)